1. Field of the Invention
The subject invention is directed to electromagnetic field sensors, and more particularly, to a blade tip measurement system that uses a marginal oscillator circuit and a heat resistant sensor assembly to generate an RF electromagnetic field in a casing of a gas turbine engine, whereby perturbation of the field by an array of rotating blade tips represents a change in susceptibility that impacts the circuit to indicate a clearance measurement between the blade tips and the engine casing.
2. Description of Related Art
In an axial flow gas turbine engine, it is desirable to minimize the clearance between the blade tips of a turbine rotor and the engine casing that surrounds the rotor. This is because excessive clearances between the blade tips and the engine casing reduce engine efficiency, and contact between the blade tips and engine casing causes damage to the engine.
The prior art includes a variety of solutions for maintaining blade tip clearance in a gas turbine engine. One such solution is a mechanical system for adjusting the radial position of the casing surrounding the rotor blades to improve engine efficiency, as disclosed in U.S. Pat. No. 5,104,287. Another solution is a mechanical system for moving the rotor disc relative to the engine casing, as disclosed for example in U.S. Pat. No. 5,330,320. An active clearance control systems that includes an actuator for moving a rotor blade assembly relative to the engine casing to maintain the minimum design clearance between the blade tips and the engine casing is disclosed in U.S. Pat. No. 7,407,369, the disclosure of which is incorporated by reference in its entirety.
These clearance control systems require a mechanism for monitoring blade tip clearance with a high degree of accuracy, under the severe environmental conditions that exist within the turbine gas path. These severe conditions include high blade tip speeds, vibration modes, high pressure fluctuations and the exceedingly high temperatures of the turbine gases, which can be as hot as 1400° C.
An example of a prior art blade tip clearance monitoring system is disclosed in U.S. Pat. No. 6,678,060 to Hayworth, which employs a group of photo-cells that monitor the position of the blade tips by detecting changes in the shape of a detected image. Another example of a prior art blade tip monitoring system is disclosed in U.S. Pat. No. 5,739,524 to Fally, which employs an optical probe that senses the distance of an object by measuring reflected radiation.
While optical measuring devices known in the art are effective for measuring blade tip clearance, they are often susceptible to the severe thermal environment of a gas turbine engine, leading to difficulties in calibration, which can result in inaccurate measurements over time.